U.S. patent application number 14/034151 was filed with the patent office on 2015-03-26 for leds with improved light extraction.
This patent application is currently assigned to General Electric Company. The applicant listed for this patent is General Electric Company. Invention is credited to Kevin Henry Janora, Jie Jerry Liu, Xiaolei Shi, Joseph John Shiang, Srinivas Prasad Sista.
Application Number | 20150084005 14/034151 |
Document ID | / |
Family ID | 52690151 |
Filed Date | 2015-03-26 |
United States Patent
Application |
20150084005 |
Kind Code |
A1 |
Sista; Srinivas Prasad ; et
al. |
March 26, 2015 |
LEDS WITH IMPROVED LIGHT EXTRACTION
Abstract
A light extraction structure that includes a composition of a
base material and a scattering material disposed within the base
material. The scattering material is a metal oxide, and the
difference between the refractive indices of the base material and
the scattering material is at least +/-0.05.
Inventors: |
Sista; Srinivas Prasad;
(Niskayuna, NY) ; Liu; Jie Jerry; (Niskayuna,
NY) ; Shi; Xiaolei; (Niskayuna, NY) ; Shiang;
Joseph John; (Niskayuna, NY) ; Janora; Kevin
Henry; (Schenectady, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
General Electric Company |
Schenectady |
NY |
US |
|
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
52690151 |
Appl. No.: |
14/034151 |
Filed: |
September 23, 2013 |
Current U.S.
Class: |
257/40 |
Current CPC
Class: |
H01L 2251/5369 20130101;
H01L 51/5268 20130101 |
Class at
Publication: |
257/40 |
International
Class: |
H01L 51/52 20060101
H01L051/52 |
Claims
1. A light extraction structure comprising: a base material with a
first refractive index; and a scattering material with a second
refractive index disposed within the base material, wherein the
scattering material is a metal oxide; wherein the difference
between the first and second refractive indices is at least
+/-0.05.
2. The light extraction structure of claim 1, wherein the light
extraction structure comprises an organic light emitting diode
(OLED).
3. The light extraction structure of claim 2, wherein the base
material has solvent resistant properties to organic solvents.
4. The light extraction structure of claim 1, wherein the
scattering material comprises particles ranging in size from 0.5
.mu.m to 5 .mu.m.
5. The light extraction structure of claim 1, wherein the base
material is a glass material with a low melting point.
6. The light extraction structure of claim 1, wherein the base
material is a polymer.
7. The light extraction structure of claim 1, comprising a
semiconductor material, wherein the first refractive index of the
base material matches a refractive index of the semiconductor
material.
8. The light extraction structure of claim 1, comprising a
semiconductor material, wherein the first refractive index of the
base material is less than a refractive index of the semiconductor
material.
9. The light extraction structure of claim 8, wherein nanoparticles
with a third refractive index are dispersed uniformly within the
base material.
10. The light extraction structure of claim 9, wherein the
nanoparticles comprise a metal oxide and range in size from 2 nm to
20 nm.
11. The light extraction structure of claim 9, wherein the base
material is a glass material, and the third refractive index of the
nanoparticles is greater than the first refractive index of the
base material by at least 0.1.
12. The light extraction structure of claim 9, wherein the base
material is a polymer, and the third refractive index of the
nanoparticles is greater than the first refractive index of the
base material by at least 0.3.
13. The light extraction structure of claim 1, wherein the light
extraction structure is planarized.
14. A light extraction structure comprising: a first layer
comprising a base material with a first refractive index and a
scattering material with a second refractive index disposed within
the base material; and a planarization layer with a third
refractive index disposed directly over the base material.
15. The light extraction structure of claim 14, wherein the
scattering material comprises a metal oxide.
16. The light extraction structure of claim 14, wherein the light
extraction structure comprises an OLED.
17. The light extraction structure of claim 16, wherein the base
material and the planarization layer have solvent resistant
properties to organic solvents.
18. The light extraction structure of claim 14, wherein the
planarization layer has a transparency of at least 90%, a haze of
less than 5%, or both.
19. The light extraction structure of claim 14, wherein the
planarization layer is a glass material.
20. The light extraction structure of claim 14, wherein the
planarization layer is an ultraviolet curable polymer or
monomer.
21. The light extraction structure of claim 14, comprising a
semiconductor material, wherein the third refractive index of the
planarization layer matches that of the semiconductor material.
22. The light extraction structure of claim 14, comprising a
semiconductor material, wherein the third refractive index of the
planarization layer is less than that of the semiconductor
material.
23. The light extraction structure of claim 22, wherein
nanoparticles with a fourth refractive index are dispersed
uniformly within the planarization layer.
24. The light extraction structure of claim 23, wherein the
nanoparticles comprise a metal oxide and range in size from 2 nm to
20 nm.
25. The light extraction structure of claim 23, wherein the
planarization layer is a glass material, and the fourth refractive
index of the nanoparticles is greater than the third refractive
index of the planarization layer by at least 0.3.
26. The light extraction structure of claim 23, wherein the
planarization layer is a polymer or monomer, and the fourth
refractive index of the nanoparticles is greater than the third
refractive index of the planarization layer by at least 0.1.
27. The light extraction structure of claim 14, wherein the first
layer is textured with micro lenses or micro cones with dimensions
less than 10 .mu.m.
28. A light emitting diode (LED) comprising: a substrate; a light
extraction structure disposed directly over the substrate and
comprising a first layer comprising a base material and a
scattering material, wherein the scattering material comprises a
metal oxide; a first electrode disposed over the light extraction
structure; a plurality of layers of semiconductor materials
disposed over the first electrode; and a second electrode disposed
over the layers of semiconductor materials, wherein the light
extraction structure is configured to reduce the amount of total
internal reflection that occurs within the semiconductor
materials.
29. The LED of claim 28, wherein the LED is an OLED.
30. The LED of claim 28, wherein the light extraction structure
comprises a planarization layer.
31. The LED of claim 28, wherein the refractive indices of each
successive layer decreases when moving from the layers of
semiconductor materials to the first electrode.
32. The LED of claim 28, wherein the first electrode comprises a
transparent anode and the second electrode comprises a cathode.
33. The LED of claim 28, wherein the first electrode comprises a
transparent cathode and the second electrode comprises an anode.
Description
BACKGROUND
[0001] In recent years, organic semiconductor devices have become
more prevalent in technologies developed for lighting and display
applications. Organic semiconductor devices are often a low cost,
high performing alternative to traditional silicon semiconductor
devices. One such organic semiconductor device is an organic
light-emitting diode (OLED). An OLED is a device that contains
organic materials that convert electrical energy into light.
[0002] Generally, OLEDs are fabricated by depositing thin films of
organic semiconductor materials in between two conductive materials
that act as electrodes. This organic material stack is then placed
between two substrates, often made of glass, and plastic with
moisture barriers, to hermetically seal the device from moisture
and oxygen.
[0003] The two electrodes provide charge carriers, either electrons
or holes, to the OLED. When an external voltage is applied, the
opposing charge carriers recombine in the organic materials and, as
a result, emit light. However much of the light produced by OLEDs
is trapped within the device. For a typical OLED, only .about.20%
light generated can escape from the substrate and optical losses
can amount to up to eighty percent of the light emitted in the
organic materials in an OLED. Typically, about thirty percent of
the light is trapped within the substrates, and another thirty
percent is trapped in the organic materials. This problem of high
optical losses also occurs in traditional light emitting diodes
(LED), as they share the same structure as OLEDs, with the
exception of using inorganic semiconductor materials.
BRIEF DESCRIPTION
[0004] Certain embodiments commensurate in scope with the
originally claimed subject matter are summarized below. These
embodiments are not intended to limit the scope of the claimed
subject matter, but rather these embodiments are intended only to
provide a brief summary of possible embodiments. Indeed, the
present disclosure may encompass a variety of forms that may be
similar to or different from the embodiments set forth below.
[0005] In one embodiment, a light extraction structure includes a
base material and a scattering material dispersed within the base
material. The base material and the scattering material have a
first and second refractive index, respectively, and the difference
between the two refractive indices is at least +/-0.05. The
scattering material is a metal oxide. The base material is
amorphous.
[0006] In another embodiment, a light extraction structure includes
a first layer that includes a base material and a scattering
material disposed within the base material. The light extraction
structure also includes a planarization layer disposed directly
over the first layer. The base material, scattering material, and
planarization layer have a first, second, and third refractive
index, respectively.
[0007] In yet another embodiment, a light emitting diode (LED)
includes: a substrate, a light extraction structure disposed over
the substrate, a transparent anode disposed over the light
extraction structure, a plurality of layers of semiconductor
materials disposed over the transparent anode, and a cathode
disposed over the layers of semiconductor materials. The layers of
semiconductor materials include: a hole injection layer, a hole
transport layer disposed over the hole injection layer, a light
emission layer disposed over the hole transport layer, and an
electron transport layer disposed over the light emission layer.
The light extraction structure includes a first layer that includes
a base material and a scattering material disposed within the base
material. The scattering material is a metal oxide. The base
material is amorphous. The light extraction structure is configured
to reduce the amount of total internal reflection that occurs
within the semiconductor materials.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0009] FIG. 1 is a cross-sectional view of a conventional OLED;
[0010] FIG. 2 is a cross-sectional view of an OLED with a single
layer light extraction structure, in accordance with one embodiment
of the present approach;
[0011] FIG. 3 is a cross-sectional view of an OLED with a single
layer light extraction structure, in accordance with another
embodiment of the present approach;
[0012] FIG. 4 is a cross-section view of an OLED with a bi-layer
light extraction structure, in accordance with one embodiment of
the present approach;
[0013] FIG. 5 is a cross-sectional view of an OLED with a bi-layer
light extraction structure, in accordance with another embodiment
of the present approach;
[0014] FIG. 6 is a cross-sectional view of an OLED with a bi-layer
light extraction structure, in accordance with another embodiment
of the present approach;
[0015] FIG. 7 is a graph displaying the efficiency of OLEDs with
and without a single layer light extraction structure;
[0016] FIG. 8 is a graph displaying the leakage current of an OLED,
an OLED with a single layer light extraction structure, and an OLED
with a bi-layer light extraction structure;
[0017] FIG. 9 is a graph displaying the efficiency of an OLED, an
OLED with a single layer light extraction structure, and an OLED
with a bi-layer light extraction structure; and
[0018] FIG. 10 is a graph displaying the surface profile of a
portion of the bi-layer light extraction structure of FIG. 6, in
accordance with an embodiment of the present approach.
DETAILED DESCRIPTION
[0019] In the following specification and the claims which follow,
reference will be made to a number of terms which shall be defined
to have the following meanings. The singular forms "a", "an", and
"the" include plural referents unless the context clearly dictates
otherwise. The term "semiconductor materials" may refer to any
material whose electron-hole recombination process results in
optical emission. The term "organic materials" may refer to small
molecular organic compounds, high molecular organic compounds,
phosphorescent materials, or polymer organic compounds. As used
herein, the term "disposed over" or "deposited over" refers to
disposed or deposited directly on top of and in contact with, or
disposed or deposited on top or but with intervening layers there
between. The term "disposed directly over" or "deposited directly
over" refers to disposed or deposited directly on top of and in
contact with and with no intervening layers there between. It
should be appreciated that the illustrated organic light emitting
diodes (OLEDs) are merely provided as an example and, accordingly,
that the embodiments described herein may be employed in any light
emitting diode (LED).
[0020] Referring now to FIG. 1, a conventional OLED 10 is an
organic semiconductor device that emits light when connected to an
external power supply. The conventional OLED 10, as shown, is a
multi-layer dielectric stack including a substrate 12, an anode 14,
a hole injection layer (HIL) 16, a hole transport layer (HTL) 18, a
light emission layer (EML) 20, an electron transport layer (ETL)
22, and a cathode 24. Although it is not shown in FIG. 1, a second
substrate 12 may be disposed over the cathode 24.
[0021] The substrate 12 is typically a glass or plastic material,
and provides a hermetic seal for the conventional OLED 10 against
moisture and oxygen. The anode 14 supplies holes to the HIL 16 and
the cathode 24 supplies electrons to the ETL 22 during device
operation. The cathode 24 may include an electron injection layer
(EIL) or may be a separate layer deposited over the EIL. One or
both of the anode 14 and the cathode 24 are made of thin
transparent conducting films such as indium tin oxide. The
conventional OLED 10 includes a transparent anode 14 through which
light is emitted during device operation, as illustrated in FIG. 1.
The HIL 16, the HTL 18, the EML 20, the ETL 22, and the EIL, if it
is separate from the cathode 24, form the semiconductor materials
26 of the conventional OLED 10. As noted above, the semiconductor
materials 26 are one or more materials whose electron-hole
recombination process results in optical emission. In particular,
the semiconductor materials 26 of the conventional OLED 10 are one
or more organic materials that may be small molecular organic
compounds, high molecular organic compounds, phosphorescent
materials, or conjugated polymers, as described above. Accordingly,
in traditional LEDs, the semiconductor materials 26 may be one or
more inorganic materials such as GaAs or ZnSe.
[0022] When the conventional OLED 10 is connected to an external
voltage source, the electrons and holes provided by the anode 14
and the cathode 24 recombine in the EML 20. This recombination
process leads to an excess of energy in the form of photons.
Although the emitted light is within the near-infrared, visible, or
near-ultraviolet portions of the spectrum, the actual wavelength of
the light is determined by the semiconductor materials 26,
specifically the amount of energy left over after successful
recombination.
[0023] However the direction in which the photons travel is
uncontrolled in the conventional OLED 10 and in traditional LEDs,
and so the amount of light which is emitted through the transparent
anode 14 and the substrate 12 is only a fraction of the total light
produced. Much of the light is trapped within the semiconductor
materials 26 the transparent anode 14 as well as the substrate 12
due to total internal reflection (TIR). TIR is a phenomenon that
occurs when light attempts to pass from one material with a
refractive index a to a second material with a refractive index b,
wherein refractive index b is less than refractive index a. If the
light strikes the boundary between the two materials at some angle
larger than or equal to a critical angle, then all of the light is
reflected.
[0024] To reduce the amount of TIR within the semiconductor
materials 26, one or more additional layers may be placed between
the semiconductor materials 26 and the anode 14. The one or more
additional layers may include a scattering material to change the
direction in which the emitted light travels. The one or more
additional layers may also have a refractive index such that the
refractive index of the LED layers slowly decreases when moving
from the semiconductor materials 26 to the anode 14. As a result,
the one or more additional layers may reduce the difference between
the refractive indices of two successive layers, which subsequently
increases the value of the critical angle and reduces the amount of
light that is reflected.
[0025] Turning now to FIG. 2, the OLED 28 with a single layer light
extraction structure 30 is illustrated. The OLED 28 is similar in
structure to the conventional OLED 10 regarding the substrate 12,
the anode 14, the cathode 24, and the semiconductor materials 26.
However, the OLED 28 includes a single layer light extraction
structure 30 that is deposited between the anode 14 and the
substrate 12, as shown in FIG. 2.
[0026] The single layer light extraction structure 30 may be a
light scattering composition, including a base material 32 and a
scattering material 34. The base material 32 may be a glass
material or an organic binder that has a first refractive index
that is high enough to match that of the semiconductor materials 26
in an LED. For example, the first refractive index is preferably at
least 1.7 to match that of most semiconductor materials 26 used in
OLEDs 28. Additionally, if the semiconductor materials 26 are one
or more organic materials, then the base material 32 may need to
have excellent solvent resistance properties to commonly used
organic solvents, such as Toluene, Acetone, Isopropanol, and
Chlorobenzene.
[0027] The scattering material 34 may be micron-size particles
ranging in size from 0.2 .mu.m to 10 .mu.m, embedded in the base
material 32. The scattering material 34 may be a crystalline metal
oxide such as, but not limited to, ZrO.sub.2, Al.sub.2O.sub.3,
TiO.sub.2, ZnO, HfO.sub.2, and HfSiO.sub.2. The scattering material
34 has a second refractive index, and the difference between the
first refractive index and the second refractive index should be at
least +/-0.05. The greater the difference between the first
refractive index and the second refractive index, the more
scattering will occur in the single layer light extraction
structure 30.
[0028] It may be desirable to use a base material 32 with a first
refractive index that is less than that of the semiconductor
materials 26, due to reduced manufacturing costs, a wider variety
of eligible materials, reduced weight, or any number of other
criteria. For example, the base material 32 in the OLED 28 may be a
spin-on-glass or polymer material with excellent solvent resistance
properties to commonly used organic solvents, and with a first
refractive index that is less than 1.7.
[0029] To raise the refractive index of the base material 32,
nanoparticles 36 may be uniformly dispersed within the base
material 32, as shown in FIG. 3. The nanoparticles 36 may be metal
oxides such as ITO, TiO.sub.2, ZnO, ZrO.sub.2, and HfO.sub.2. The
nanoparticles 36 range in size from 2 nm to 20 nm such that they
scatter a minimal amount of light compared to the scattering
material 34. The nanoparticles 36 have a third refractive index. If
the base material 32 is a spin-on-glass material, then the third
refractive index should be at least the first refractive index+0.1.
If the base material 32 is a polymer material, then the third
refractive index should be at least the first refractive
index+0.3.
[0030] While the single layer light extraction structure 30 does
decrease the TIR and increase the light output of the OLED 28, the
OLED 28 may exhibit a much higher amount of leakage current
compared to the conventional OLED 10. This is because the single
layer light extraction structure 30 may have many micron size
defects due to fabrication. These defects lead to increased
shorting and reduced yield of the OLED 28.
[0031] To reduce the amount of leakage current, an OLED 38 uses a
bi-layer light extraction structure 40, as shown in FIG. 4. The
bi-layer light extraction structure 40 is the single layer light
extraction structure 30 with a planarization layer 42 deposited
directly over it. The planarization layer 42 smoothes the rough
surface of the single layer light extraction structure 30, reducing
the amount of leakage current and subsequently increasing the yield
of the OLED 38. The planarization layer 42 also simplifies the
manufacturing process, as it provides a smooth surface over which
the semiconductor materials 26 may be deposited.
[0032] In general, the planarization layer 42 should have a high
refractive index, high transparency (at least 90%), and low haze
(less than 5%, preferably less than 1%). If used in an OLED, it
should also have excellent solvent resistance properties to
commonly used organic solvents, similar to the single layer light
extraction structure 30.
[0033] The planarization layer 42 may be a spin-on-glass material
with a fourth refractive index that matches that of the
semiconductor materials 26, as shown in FIG. 4. However, the
planarization layer 42 may also be a spin-on-glass material or
ultraviolet (UV) curable polymer or monomer with a fourth
refractive index that is less than that of the semiconductor
materials 26. If so, then the nanoparticles 36, with a third
refractive index, may be added to the planarization layer 42 to
increase the refractive index of the planarization layer 42, as
shown in FIG. 5. The size of and type of materials used for the
nanoparticles 36 is the same as listed for the single layer light
extraction structure 30. If the planarization layer 42 is a
spin-on-glass material, then the third refractive index should be
at least the fourth refractive index+0.3. If the planarization
layer 42 is a UV curable polymer or monomer, then the third
refractive index should be at least the fourth refractive
index+0.1.
[0034] The OLED 38 may include a single layer light extraction
structure 30 that is intentionally textured, as shown in FIG. 6.
The single layer light extraction structure 30 is textured with
micro lenses or micro cones with dimensions less than 10 p.m. The
existence of peaks and troughs in the single layer light extraction
structure 30, as opposed to a more uniform surface, may lead to
additional scattering due to the variety of angles at which light
may strike the surface. The textured single layer light extraction
structure 30 may be manufactured in the same manner as one or more
of the embodiments described above.
Examples
Manufacture of Glass Substrate with Scattering Layer
[0035] A solder glass slurry was prepared in a 60 ml plastic
Nalgene bottle. 0.3291 g (1.5% of final mass) 1 micron zirconium
(IV) oxide (Alfa stock No. 40140) was added to 21.63 g Schott 8465
solder glass (75% total solids) & 7.368 g Bush terpineol (25%
liquid). The Schott 8465 solder glass were 5 micron particles
(Schott's K5 grind) and used as received. The resulting mixture was
hand mixed briefly with a stainless steel spatula and then milled
to break up agglomerates that are noticeable as large chunks during
tape casting. About twenty 1/4'' diameter and five 1/2'' diameter
cylindrical yttria-stabilized zirconia milling media were added to
the slurry. The 1''.times.1'' or 3''.times.3'' soda-lime glass
substrates were cleaned by rubbing them with a 2-propanol soaked
cleanroom wipe and a 2-propanol rinse and then blow dried using a
nitrogen gun.
[0036] Two layers of 50 microns thick Scotch tape were then applied
on either side of the soda-lime glass substrates to create a gap of
100 microns. A small blob of the slurry was then applied at one end
of the substrate. A razor or 2''.times.3'' microscope slide edge
was dragged across the substrate at a 45.degree. angle to create a
100 micron thick wet slurry film. The approximate speed of blade
was .about.2 mm/sec. Any excess slurry was wiped off the edges and
at the bottom to prevent the substrate from sticking to the
stainless steel plate during firing.
[0037] The scotch tape was removed before drying the films in open
air on a hot plate at 125.degree. C. for 10 minutes. The dried
substrate was then placed on an oxidized 321 stainless steel plate
and covered with a stainless steel sheet placed 1 cm above the
surface of the coated substrate or was placed in a stainless steel
bag. The stainless steel plate was then inserted into a Lindberg
type 51848 box furnace, which was heated to 450.degree. C. at a
rate of 100.degree. C./min. After maintaining the temperature of
the furnace at 450.degree. C. for 10 minutes, the temperature was
slowly increased from 450.degree. C. to 550.degree. C. at 5.degree.
C./min. The substrates were heated at 550.degree. C. for 2 hours.
To cool the substrates, the furnace was turned off and the
substrates sat overnight in the furnace with the furnace door
closed. After 24 hours at room temperature and humidity, the
substrates were refired in the same furnace with a moderately slow
increase of 5.degree. C./min to a temperature of 650.degree. C. and
then held for 2 hours. Finally the substrates were again cooled
down overnight with the furnace off.
Fabrication of OLEDs with and without a Single Layer Light
Extraction Surface
[0038] OLEDs were fabricated on plain glass substrate that was used
as control device (Device A) and with a single layer light
extraction surface (Device B). We used solder glass layers with
1.5% concentration of 1 .mu.m Zirconia particles (GOG:ZrO.sub.2) as
the single layer light extraction structure.
[0039] Next all the substrates were coated with ITO film by
sputtering. Substrates were cleaned sequentially using detergent
solution, DI Water, Acetone and Isopropanol. The substrates were
then blown dry using a nitrogen gun and a ten minute UV ozone
treatment. CH8000 was used as a hole injection material and was
spin coated on cleaned substrates at 5000 rpm to achieve 50 nm
thick films that were subsequently baked at 120.degree. C. for 10
min in air. The parts were then transferred into an inert
atmosphere to coat the subsequent organic layers. A hole transport
layer was spin coated at 2500 rpm from 0.5 wt. % solution of TFB
polymer in Toluene and was baked at 200.degree. C. for 60 minutes.
A thick emissive layer (200 nm) of a fluorescent green polymer
(LEP1304) was obtained by spin coating at 1400 rpm from 2.0 wt. %
solution in p-Xylene. The resulting films were baked at 135.degree.
C. for 15 minutes. In the final step the electron injection layer
(NaF-38 .ANG.) and the top metal contact (Al-1200 .ANG.) were
deposited using thermal vapor deposition at 10.sup.-6 torr
deposition pressure.
[0040] FIG. 7 shows the relative improvement in current efficiency
of a green polymer OLED when GOG:ZrO.sub.2 is used as a single
layer light extraction structure. As shown below in Table 1, there
is a thirty percent improvement in the external quantum efficiency
(EQE) of the OLED with a single layer light extraction structure
(device B) relative to a conventional OLED as the control (device
A). This improvement in efficiency occurs because the single layer
light extraction structure effectively extracts the device modes
out into the air.
TABLE-US-00001 TABLE 1 At Brightness of 1000 Candela/m.sup.2 Lumens
External Drive per Quantum At Current Density Voltage Watt
Efficiency Watts/Watts of 10 mA/cm.sup.2 ID (DV) (LPW) (EQE) (W/W)
DV cd/m2 LPW Control (Device 9.57 5.5 5.0% 1.2% 11.25 1651 4.6 A)
GOG:ZrO.sub.2 8.39 8.4 6.6% 1.7% 10.73 2233 6.5 (Device B)
Manufacture of Glass Substrate with Scattering and Planarization
Layers
[0041] As a result of processing a GOG:ZrO.sub.2 single layer light
extraction surface on top of a soda lime glass substrate, the
surface of the GOG:ZrO.sub.2 layer has many micron size bumps.
These micron size particle defects lead to shorting of OLEDs and
hence reduce the yield of OLEDs. FIG. 8 shows the leakage current
values of green polymer OLEDs fabricated on a plain glass substrate
and a glass substrate with a GOG:ZrO.sub.2 single layer light
extraction structure. Several of the devices with a single layer
light extraction structure have a high leakage current
(>10.sup.-2 mA/cm.sup.2).
[0042] In order to reduce the leakage current of OLEDs and hence
improve the yield, a planarization layer was deposited over the
rough surface of the GOG:ZrO.sub.2 single layer light extraction
structure. A 10 .mu.m thick UV-curable acrylate layer was deposited
over the GOG:ZrO.sub.2 layer. Adding the SR492 planarization layer
reduced the amount of leakage current, as shown in FIG. 8. The
leakage current was reduced to less than 10.sup.-4 mA/cm.sup.2,
thereby improving the reliability of OLEDs.
Manufacture of Glass Substrate with Textured Scattering Layer
[0043] Grit blasting of soda-lime glass was done using a
grit-blaster. 50 .mu.m Mintec Quartz or 30-70 micron PTI Powder
Technology's Arizona Test Dust was used as the grit-blasting media
that was fed in at 5 grams/minute with 30 psi air at 25 SLM (42
SCFH) through a 64 mil ID alumina tube nozzle. The glass surface
was typically kept at 5-10 mm from the nozzle tip. The nozzle was
rastered across the surface roughening 1 cm.sup.2 in about 10
seconds. Afterwards, a 10 minute ultrasonic in DI water was done to
remove the residual glass dust. A brief toothbrush scrubbing
followed by a DI rinse and a 80.degree. C. hot plate drying
resulted in cleaner grit-blasted surfaces. Surface roughness was
measured using Tenco stylus profilometer and the surface profile is
shown in FIG. 10. The root mean square (RMS) roughness of the grit
blasted soda-lime microscope slide glass is around 1.0 .mu.m and
peak-to-trough value was about 4 .mu.m.
[0044] One or more of the disclosed embodiments, alone or in
combination, may provide one or more technical effects useful for
designing and manufacturing LEDs used in display and lighting
applications. Certain embodiments may allow for increased
efficiency in LEDs. For example, the present single layer light
extraction structure reduces the amount of light trapped within the
semiconductor materials of an OLED due to TIR, compared to existing
OLED technology. The present bi-layer light extraction structure
not only reduces the amount of TIR, but also reduces the amount of
leakage current to a level similar to, or better than, that of
existing OLED technology. The technical effects and technical
problems in the specification are exemplary and not limiting. It
should be noted that the embodiments described in the specification
may have other technical effects and can solve other technical
problems.
[0045] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to practice the invention, including making and
using any devices or systems and performing any incorporated
methods. The patentable scope of the invention is defined by the
claims, and may include other examples that occur to those skilled
in the art. Such other examples are intended to be within the scope
of the claims if they have structural elements that do not differ
from the literal language of the claims, or if they include
equivalent structural elements with insubstantial differences from
the literal languages of the claims.
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